This invention relates to offshore oil well operations and is specifically directed to a new and improved marine riser system for connecting marine risers extending from a well located on the ocean floor (subsea) to the platform of a vessel. Marine risers are sections of pipes sometimes called "conductors" or "casings" connected together as a "string" of "risers" and the vessels are suitably outfitted "ships" or "semi-submersibles", also referred to as "rigs".
In particular, the object of this invention is to provide a new and improved upper marine riser package which includes a self-tensioning riser slip joint and other equipment connectable, as a modular unit, to the upper end of the riser string and capable of being lowered through the rotary table on the vessel. Modularity of the package also facilitates retrieval for repair and maintenance.
This upper marine riser package may, in one application, eliminate all of the equipment of the conventional riser tensioning system, i.e., guide sheaves, wire ropes, and riser tensioners, as well as all of the necessary replacement equipment required to be stored on board the rig due to frequent breakage, such as extra reels of wire rope, extra sheaves, etc. In another application, this invention may be used where there are additional load requirements, such as in deeper water drilling, by providing additional load capabilities to the conventional riser tensioning systems. Thus, this invention can be used on newly constructed rigs either alone or as an addition to conventional tensioning systems or can be used as a retrofit for existing rigs to eliminate the conventional riser tensioning systems or as an addition to the conventional riser tensioning systems.
The need for slip joints and tensioning devices for marine risers supported from a platform of a vessel to a subsea well are amply described in any number of prior art patents such as in the U.S. Pat. No. 3,933,108 to Baugh directed to tanks to make the riser sections substantially bouyant, in the U.S. Pat. No. 4,367,981 to Shapiro directed to a slip joint form of a piston-cylinder configuration mounted at the upper portion of the riser, in the U.S. Pat. No. 3,353,851 to Vincent and in the U.S. Pat. No. 3,211,224 to Lacey, one of which shows a slip joint at the upper end of the riser string and the other at the lower end of the riser string.
However, none of the prior art patents cited above, as examples, suggest an upper marine riser package as an integral unit which could be made up and run, or retrieved for repair and maintenance, through the rig rotary table. Further, none of these patents suggest a package which, when used alone, would eliminate the aforementioned conventional riser tensioning systems or, alternatively, when used with such conventional systems, would provide additional load capabilities for longer and longer strings of risers for deeper and deeper subsea operations.